the protagonist often ends up pursuing a relationship with the Mary Sue female lead who has practically no personality, despite a (by Western standards anyway) more attractive and interesting romantic interest with whom they share more in common. Anna is like a blank slate. She only shows up when the protagonist needs something and her primary personality traits are "genius and female." Nightingale consistently shows more personality, nuance, and maturity. She's a far more complex character whose internal motivations extend beyond pleasing Roland because he rescued her that one time, even when she does find herself hopelessly attracted to him she realizes she has no chance and leads to its own kind of character development. I get that romance doesn't work that way but it seems like the author has spent more time developing a strong female lead that complements the protagonist in a meaningful way and then decided it was easier to phone it in by using stock character A for the romantic subplot. Like what? What do Anna and Roland have to talk about with each other than how much they like each other and how much heat Anna should apply to iron to solder an electromagnet? It's a weird take on a relationship that's primarily defined not by the mutual attraction of equals but by someone whose entire life literally revolves around the other's. It's kind of creepy since if Anna were to reject Roland she literally risks death. Despite her powers she doesn't have the skills or experience to survive on her own, she's completely dependant on Roland emotionally and physically. Roland struggles with this for all of 5 minutes before he decides that newly 18 y/o child with no personality and complete dependence on him is a more interesting and exciting romantic partner than the more attractive (to me) woman with whom he actually shares goals and can hold a conversation with for more than 2 minutes because she actually has something in her head. They might even get into some sort of ideological conflict that would force the protagonist to think about his actions because he values his relationship with someone who is his equal rather than someone who will huff for 5 minutes and call him whatever the Chinese equivalent of "baka" before smiling sweetly and forgiving him because he spent too much time thinking about the well-being of his people or because his first instinct was not to confide in her when he's distraught over some tough decision she would be absolutely unable to provide any valuable insight into.
tl;dr- The series goes downhill due to the author copping out and pairing up Roland and Anna when Nightingale is a far superior character in both her compatibility with Roland and her overall characterization whereas Anna is a background character that gets far too much focus and is inexplicably thrust into a relationship with the protagonist for no other reason than reader wish fulfilment because she's a cute and pure teen whose only motivations are pleasing Roland (because she will literally die if she puts her out) and wasting air.